As is well known, the compressor case of a gas turbine engine powering aircraft is subjected to severe pressure and temperature loadings throughout the engine operating envelope and care must be taken to assure that the components remain concentric maintaining relatively close running clearances so as to avoid inadvertent rubs. Inasmuch as the engine case is thin relative to the rotor and stator components in the compressor section, it responds more rapidly to temperature changes than do other components. This is particularly true during periods of transient engine performance. Typical of these transients are throttle chops, throttle bursts, and the like. Obviously it is customary to provide sufficient clearances during these transients to assure that the rotating parts do not interfere with the stationary parts.
The problem becomes even more aggravated when the engine case is fabricated in two halves (split case) which is necessitated for certain maintenance and construction reasons. Typically, the halves are joined at flanges by a series of bolts and the flanges compared to the remaining portion of the circumference of the case is relatively thick and hence does not respond to thermal and pressure changes as quickly as the thinner portion of the case. The consequence of this type of construction is that the case has a tendency to grow eccentrically or out of round.
In order to attain adequate roundness and concentricity to achieve desired clearance between the rotating and non-rotating parts, I have found that the segmented stator vanes tied to a full hoop case for the highest stages of a multiple stage compressor of a gas turbine engine enhanced the structural integrity sufficiently to meet the engine's requirements. Since the stator components, i.e., stator vanes and outer air seals, are segmented the problem was to assure that the compressor maintained its surge margin notwithstanding the fact that the outer case would undergo large deflection at acceleration and deceleration modes of operation. The cavity that exists between the outer case and the inner case formed by the segmented stator components, being subjected to pressures occasioned by the flow of engine air through the various leakage paths, presented a unique problem. In the event of a surge, which is a non-designed condition, the pressure in the gas path would be reduced significantly. Because the air in the cavity is captured and cannot be immediately relieved, it would create an enormous pressure difference across the stator components, cause them to distort, with a consequential rubbing of the compressor blades, and a possible breakage.
In order to withstand this pressure loading and yet achieve the roundness and clearance control of the stationary and rotating components it was necessary to incorporate a mechanism that would tie the outer case to the segmented stator components. While it became important to assure that this rubbing did not occur, particularly where severe rubbing could permanently damage the blades and/or rotor/stator during surge, the mechanism that is utilized must be capable of withstanding this enormous load, yet be insensitive to fatigue.
Moreover, in order to achieve roundness and maintain close tolerance between the tips of the blades and outer air seal it is abundantly important that the components subjected to high thermal and load differentials do not allow the outer and inner cases to grow eccentrically. To this end the aft end of the stator is supported by a bulkhead or a back bone that is formed from a relatively straight shaped annular support that is attached to the inner segmented case and the outer case. The bulkhead or backbone is attached in such a way that the inner case floats axially and circumferentially while being restrained radially. While this arrangement enhances the control of the clearances between the tips of the blades and outer air seal, it is only a portion of the support necessary for the stator.
While the design of the aft end of the engine case support structure as described above provides axial and circumferential freedom, it is also necessary to provide other means to allow axial and circumferential movement at the forward end of the stator structure that is supported by the full hoop engine case. In accordance with this invention, I have provided a "tongue and groove" arrangement that provides the radial restraint and allows each segment to grow thermally in the axial and circumferential direction.
This configuration also facilitates the assembly and disassembly of the full hoop over a segmented stator and drum rotor. As mentioned above, the stator vanes are segmented and includes a portion of the three rows of vanes in a single casting. When assembled, the segments are mounted end-to-end to form an inner ring-like case and three circumferential rows of spaced vanes.